The present invention relates to the field of optical joint transform correlators. Joint transform correlators (JTC) can be used to match an input image being viewed in real time with a plurality of reference images. See U.S. Pat. No. 4,357,676 issued to Hugh Brown, and U.S. Pat. No. 4,695,973 issued to F. T. S. Yu.
It has been shown previously that binary joint transform correlators can produce very good correlation performance. See B. Javidi and C. J. Kuo, "Joint Transform Image Correlation using a Binary Spatial Light Modulator at the Fourier Plane," Applied Optics, Vol. 27, No. 4, 66-665 (1988); and see B. Javidi and S. F. Odeh, "Multiple Object Identification by Bipolar Joint Transform Correlation," Optical Engineering, Vol 27, No. 4, 295-300 (1988). The binary JTC uses nonlinearity at the Fourier plane to binarize the Fourier transform interference intensity to only two values, +1 and -1. The performance of the binary JTC has been favorably compared to that of the classical JTC, (C. S. Weaver and J. W. Goodman, "A Technique for Optically Convolving Two Functions," Applied Optics, Vol. 5, No. 7, 1248-1249 (1966)) in the areas of light efficiency, correlation peak to sidelobe ratio correlation width, and cross-correlation sensitivity. The motivation for binarizing the interference intensity has been the good correlation performance obtained by binary phase-only filter-based optical correlators. See J. L. Horner and P. D. Gianino, "Phase-only matched filtering," Applied Optics, Vol. 23, No. 6,812-816 (1984); J. L. Horner and J. R. Leger, "Pattern recognition with binary phase-only filters," Applied Optics, Vol. 24, No. 5, 609-611 (1985); and J. L. Horner and H. 0. Bartelt, "Two-bit correlation," Applied Optics, Vol. 24, No. 18, 2889-2893 (1985).